1. Field of the Invention
The present invention relates to a position detecting device for obtaining displacement information of a movable member and more particularly, to a position detecting device for obtaining displacement information suitable for slowly moving an optical head or an information recording medium in, e.g., an optical information recording/reproducing apparatus.
2. Related Background Art
FIG. 1 is a block diagram showing a main portion of a conventional optical disk apparatus.
Referring to FIG. 1, light emitted from a laser diode 20 is collimated by a collimation lens 19, and the collimated light ray is reflected by a mirror 13 through a polarizing beam splitter 44 and is focused on a recording layer of an optical disk 10 through an objective lens 12.
The reflected light from the optical disk 10 is reflected again by the mirror 13 through the objective lens 12. The reflected light is then reflected downward by the polarizing beam splitter 44. One portion of the light partially reflected by a half mirror 15 is detected by a photodetector 18 through a lens 17, and the detected optical information is input to a radio-frequency (RF) signal amplifier 23. Another portion of the light passing through the half mirror 15 is detected by a four-split beam detector 16. An output from the detector 16 is input to a position error signal processing circuit 25.
An output from the position error signal processing circuit 25 is input to a focusing/tracking servo circuit 21, and an output from the servo circuit 21 is used to finely adjust the objective lens 12.
A voice coil motor (VCM) servo circuit 24 controls an optical head 11 to move the head to a position near a desired track and to stop the head on the desired track in accordance with track selection data from a microprocessor unit (MPU) 27. The MPU 27 controls a spindle motor 29 for rotating the optical disk 10 by using a spindle motor servo circuit 26 and an encoder 28. The MPU 27 also controls emission of light from the laser diode 20 through a laser driver 22.
A linear encoder shown in FIG. 2 is arranged between the optical head 11 and a housing 45. A movable slit member 31 is engaged with the optical head 11 and is moved together with the optical head 11. A stationary slit member 30, light-emitting diodes (LEDs) 32 and 33, and phototransistors (PTr) 34 and 35 are fixed on the housing 45. The positional relationship between the movable slit member 31, the stationary slit member 30, the LEDs 32 and 33, and the PTrs 34 and 35 is illustrated in FIG. 3. That is, the LED 32 opposes the PTr 34, and the LED 33 opposes the PTr 35. The phase of the fixed slit between the LED 32 and the PTr 34 is shifted from that of the fixed slit between the LED 33 and PTr 35 by 90.degree.. Outputs from the PTrs 34 and 35 are input to current-voltage converters 36 and 37, and outputs from the current-voltage converters 36 and 37 are partially input to inverting amplifiers 38 and 39, as shown in FIG. 4, to obtain four-phase alternating signals phase-shifted from each other by 90.degree. as shown in FIG. 5.
In order to continuously record or reproduce information, a spiral groove (track) called a pregroove is formed in the optical disk beforehand to perform tracking guidance. The optical head, for example, is controlled to be stopped at point A of the first phase shown in FIG. 5, and the position of the objective lens is controlled to trace the spiral track.
The objective lens is moved by an actuator (not shown) in the tracking direction. Since the range of displacement of the objective lens is limited, the entire optical head is moved to control the objective lens within displacement range.
FIG. 6 is a conventional position control block diagram of an optical head using a voice coil motor. A difference between a target value and the actual displacement of the optical head, as determined from a position detecting device 40 using the linear encoder as described above, is input to a phase compensator 41 . A predetermined amount of current is supplied to a voice coil motor 43 through a driver 42, thereby moving the optical head. Since the voice coil motor 43 does not normally have stress such as a spring bias in a movable direction, when the motor current is zero, the optical head is stopped at an arbitrary position. For this reason, when the target value in FIG. 6 is set to be 0 (V), the position of the optical head is controlled to be at point A of the first phase.
During continuous recording/reproduction, when the objective lens traces the spiral track and is moved toward the center of the disk, the entire optical head is moved toward the center of the disk to bring the objective lens to the center of operation.
For this reason, when the position of the optical head is to be at the A point of the first phase in FIG. 5, the position control output from the linear encoder is changed from an output of the first phase to an output of the second phase. The optical head reaches the D point in the order of A, B, C, and D as shown in FIG. 5. If the optical head must be at the center of the optical disk, the position control output from the linear encoder is changed from the output of the second phase to an output of the third phase under the condition that the position of the optical head is controlled to be at the position D. Then, the optical head reaches the G point in the order of D,E, F, and G. In this manner, the optical head is sequentially moved.
However, when the operating points are changed in the orders of A, B, C, and D, and D, E, F, and G, currents corresponding to these orders are supplied to the voice coil motor. In particular, the operating points are discontinuous in the orders of A to B and D to E, and accelerations are increased. Therefore, tracking servo is adversely affected such that control errors occur and servo synchronization is out of step for instance.
When the optical disk apparatus receives an external impact, causing vibrations in the optical head feed direction, the optical head is braked to a control position by the position control. If the control gain is kept unchanged, resistance to impact and vibrations is associated with a detection region of the position detecting device. Assume that the external impact acts in the optical head feed direction while the position of the optical head is controlled to the D point of the second phase in FIG. 5. The operating point is shifted to the left and right along a curve of the second phase by the impact force. At this time, during movement of the optical head in an order of D, C, and B, or D, H, and I, the amplitude is increased with an increase in displacement, so that a restoration force is increased accordingly. However, during the displacement in an order of B, L, and M, or I, J, and K, the restoration force is decreased. When the optical head is moved over the M point to the left or over the K point to the right, it falls within a positive feedback region and therefore is quickly moved to the N point away from point D of the curve of the second phase by one period and to a point (not shown) to the right of point O by one step. This state inconvenient, because tracking servo control is adversely affected, and a stop position of the optical head cannot be determined by position control.